The present invention relates to gas turbine engines, and more specifically, to compensating for measurement error using control logic.
Gas turbine engines include sensor and actuator position feedback used for control and diagnostics. The term sensor, as used herein, refers to engine mounted measurement devices. Values from the sensors are validated and then used implicitly by the control and diagnostic logic. The true values for the pressures, temperatures, actuator positions, speeds, and other engine variables, however, differ from the measured values. The difference between the true value and the measured value is referred to as measurement error.
Measurement errors are generally attributed to transducer inaccuracy, physical differences such as valve characteristics, and gas path profile effects. Measurement error due to transducer inaccuracy primarily results from manufacturing tolerances and signal conditioning errors. Measurement error due to valve characteristics results from geometry variations due to manufacturing tolerances and inaccuracies in valve position to flow relationships.
Measurement errors due to gas path profile effects are a combination of physical tolerances in the sensor and the sensor installation into the engine. The engine installation introduces dimensional variability, such as immersion depth and alignment, which can lead to differing sensing locations within the gas path temperature and pressure profile. Profile measurement error is usually much larger at aft stages of an engine since combustion and gas mixing are significant factors in such measurement error.
An absolute level of measurement error depends on an accuracy specification used in device design, the type of device, and the device utilization. For example, a pressure transducer critical for engine control typically must be much more accurate than a condition monitoring sensor for optional equipment that is not flight critical. Once installed, measurement error often is assumed to be consistent with respect to sign (i.e., positive or negative error). However, error magnitude will vary with operating conditions, e.g., a larger measurement error at high power versus idle.
Since an engine controller uses indicated values from the sensors, the effects of measurement error are accommodated in an overall engine control law design process. Typically, control laws are designed to include margins for a worstcase measurement error. While this approach provides a safe margin of operation for a worst case engine, this approach also results in larger than necessary margins for all other engines.
The present methods and systems, in one aspect, reduce conservatism inherent in selecting a worst-case measurement error by reducing, or removing, the effect of measurement error so that an actual engine operates closer to optimum. In an exemplary embodiment, the method includes the steps of identifying an engine operation (e.g., acceleration) that is less than optimal, estimating the measurement error associated with the less than optimal operation, adjusting the engine control logic based on the measurement error estimate, and reassessing the engine operation. The method is implemented, in one form, in an engine controller by programming a control processor to execute the above described method.
The above described method provides the advantage that measurement error adjustments are made depending upon the characteristics of each engine rather than making a same adjustment for all engines of a particular type or model. In addition, an engine with below nominal operating characteristics can be returned to nominal operation without replacing sensors or other related hardware, which results in increased time on-wing, lower acceleration time, and reduced operating temperatures.